Platforms and scaffolding are used for a variety of purposes including painting, welding and sandblasting. The construction of such devices differs for different applications and for different buildings or other structures with which they are to be used. Worker platforms for cylindrical fuel storage tanks and the like are typically vertically adjustable block and tackle arrangements with some provision being made for moving the platform around the tank. Examples of traveling devices for positioning workers adjacent storage tanks and similar structures are given in U.S. Pat. Nos. 1,101,839; 1,421,609; 3,114,433; 3,537,545; and 3,837,429.
Conventional traveling platforms for positioning workers at convenient stations about storage tanks have framework in the form of a boom or arm that is secured to the top center of the tank and extends radially outward across the top to a point beyond the sidewall of the tank. A stage made up of planking or similar materials is suspended by a block and tackle arrangement from the outer end of the arm. The block and tackle fitting provides selective vertical positioning. A carrier wheel or other rolling mechanism supports the arm for travel about the periphery of the tank top. The wheel provides selective circumferential positioning and, together with the block and tackle fitting, enables a worker to be positioned at any number of locations about the tank. Such traveling worker platforms are particularly useful in connection with performing construction and maintenance operations on fuel storage tanks which are surrounded by fire and spill retention walls that limit the use of ground supported devices.
A problem with the conventional block and tackle structures is, however, their instability. This is especially troublesome for operations such as sandblasting where high pressure hose nozzles must be manipulated. There is a tendency for the whole stage to swing or sway outwardly in pendulum fashion away from the tank when pressure is applied and for the ends of the stage to twist or swivel vertically about its center. Difficulty may also be encountered in keeping the stage floor level, preventing its tilting or leaning either forward or backward, or to one end. This lack of stability restricts the sandblasting procedure and may also interfere with high volume paint spraying.